The invention relates to the general field of MEMS with particular reference to filling deep trenches with oxide and with the formation of silicon beams.
The fabrication of MicroElectro Mechanical Systems (MEMS) on silicon substrates often necessitates electrical isolation between various suspended. mechanical structures that need to be electrically actuated. In a previous patent (U.S. Pat. No. 5,930,595), we disclosed a deep trench isolation process for sensors and actuators application. The isolation between microstructure and surrounding silicon was realized by oxide filled trenches. That process is suitable for deep trench isolation of up to 10-12 microns. However for higher sensitivity, there is a need for much deeper trenches (40-45 microns). This makes it necessary to have a deeper trench isolation process.
The principal limitation imposed by the process described in U.S. Pat. No. 5,930,595 is illustrated in FIG. 1a. Seen there is silicon body 12 in which deep trenches 104 have been formed. Through use of a release etch, the trench bottoms were widened to form expanded regions 103 which penetrated one another, thereby connecting all trenches and forming silicon islands 110 (FIG. 1b).
The filling of the trenches is illustrated in FIG. 1b. First the trench walls were lined with thermal oxide layer 106. Then filling was completed with the deposition of an oxide layer (TEOS) by means of LPCVD (low pressure chemical vapor deposition). This resulted in the formation of voids such as 109, whose presence is undesirable because of potential reliability problems. The void formation problem increases in severity with increasing trench depth.
A routine search of the prior art was performed. The following references of interest were found:
In U.S. Pat. No. 6,051,866, Shaw et al. show a sensor-process with initial steps similar to the present invention. Muenzel et al in U.S. Pat. No. 5,723,353 teach another sensor isolation system. Sakai et al. (U.S. Pat. No. 5,549,785) also describe an isolation process. Sparks et al. in U.S. Pat. No. 5,427,975, teach a method for filling trenches with polysilicon but said polysilicon is not oxidizable, being inserted in order to make electrical connections. In U.S. Pat. No. 5,426,070, Shaw et al. discuss the formation of beams in a silicon body by using trenches followed by a release etch step. These trenches do not get filled with oxide. MacDonald et al. (U.S. Pat. No. 5,393,375) show a similar beam formation process.
It has been an object of the present invention to provide a process for filling a deep trench in a silicon body with silicon oxide.
A further object of the invention has been that said filling process be free of void formation within the filler oxide.
A still further object has been to apply said process to the formation of embedded silicon beams that are surrounded by silicon oxide.
Another object of the invention has been to apply said process to the formation of silicon beams that are partly cantilever and partly embedded in silicon oxide.
These objects have been achieved by means of a process of deep trench etch and silicon undercut release etch by using oxide spacers to protect the silicon beam sidewalls during release etch. An oxide layer is then formed followed by deposition of a pre-determined thickness of polysilicon which is then thermally oxidized. The polysilicon layer inside the trenches gets fully oxidized resulting in void-free trench isolation. This process creates a silicon island on three sides leaving the third side for interfacing with the sensor/actuator beams. The sensor/actuator is formed by a similar process of deep trench etch and release etch process on the same substrate. These suspended beams of the sensors and actuators are bridged with the silicon islands from the fourth side. The above process finally results in suspended silicon beams connected to electrically isolated silicon islands called anchors.